1. Field of the Invention
The present invention relates generally to the field of exhaust gas recirculation and engine exhaust braking. Specifically, the invention relates to a method and system for combining exhaust gas recirculation and exhaust braking using single valve actuation.
2. Description of the Background Art
Exhaust braking is an engine operating mode wherein the engine is reconfigured during operation to provide a braking effect to a vehicle. This may be desirable or necessary when regular wheel brakes are inadequate to provide complete braking. An example is a need for powerful and prolonged braking operations on steep grades, such as on mountain roads. Exhaust braking finds particular applicability on large vehicles having high wheel weights and correspondingly high momentum, and where conventional wheel brakes may fade or fail under high loading conditions or under prolonged use.
An engine brake works by opening exhaust valves at or near the end of the compression stroke of an associated cylinder. During the compression stroke of an engine, the air in a cylinder is compressed, requiring a work input by the engine. In normal engine operation, the combustion or expansion stroke follows the compression stroke and recoups the work expended during the compression stroke. The opening of the exhaust valve near the end of the compression stroke means that no expansion of the compressed air occurs, with the air being exhausted from the engine (preferably, fuel is not injected into the engine during exhaust brake operation so that fuel is not passed through the engine unburned). The net result is that during exhaust brake operation the engine is absorbing power and not generating power. The engine exhaust brake is therefore an efficient braking system that can be used as a supplement to or a substitute for conventional wheel brakes, and may be used for repeated and extended braking operations.
Exhaust brakes may use special components, or may be realized using existing valve train components. Generally, exhaust braking requires components that can actuate (open) an exhaust valve independent of the normal valve train operation, under control of an exhaust brake system. Related art exhaust brake systems have included separate independent camshafts, rocker arms, or actuators to perform actuation of exhaust valves for exhaust braking. Related art devices have in the past actuated multiple exhaust valves in unison. This is of course the simplest operation conceptually, but simultaneous opening of both exhaust valves of a cylinder during exhaust braking has drawbacks.
The force required to open multiple valves is higher than the force required to open a single valve imposing a greater load upon the actuation components. The design for an exhaust brake assembly having single valve actuation is disclosed in Bartel et al., U.S. Pat. No. 6,234,143 B1 (May 22, 2001), the disclosure of which is incorporated herein by reference.
The exhaust brake assembly disclosed in Bartel ""143 employs an engine exhaust brake assembly capable of opening a single valve of an exhaust valve pair. The exhaust brake assembly includes a rocker arm having a camshaft force receiving portion on a proximal end of the rocker arm for receiving a force applied by a camshaft, a valve actuation contact portion on a distal end of the rocker arm and a pivot point located between the proximal and distal ends. An exhaust valve pair, including a first valve and a second valve, is provided with valve stems for use in valve actuation. The first valve is closer to the pivot point of the rocker arm and inside the valve actuation contact portion of the rocker arm. A valve bridge extends across the valve stems.
Further, the valve bridge has a contact portion located between the valve stems, and corresponds to and contacts the valve actuation contact portion of the rocker arm. The valve bridge actuates the exhaust valve pair when the valve actuation contact portion of the rocker arm exerts a force upon the valve bridge as the rocker arm pivots in operation. An exhaust brake actuator formed between the pivot point and the distal end of the rocker arm includes an actuator piston having a retracted position and an extended position. The first valve of the exhaust valve pair may be opened by extension of the actuator piston of the exhaust brake actuator while the valve opening actuation portion of the rocker arm is out of contact with the central contact portion of the valve bridge.
The opening of only one exhaust valve during exhaust braking reduces the load imposed on the pushrod by fifty percent for any given cylinder pressure when compared to a two valve exhaust braking operation. The imposed load is even further reduced since the first exhaust valve (e.g., the valve closest to the rocker shaft) is the valve being opened. Accordingly, the engine braking performance can be optimized without being limited by cylinder pressures, and with less compliance in the valve train.
Another way of increasing the braking power of exhaust brakes is to perform exhaust gas recirculation in combination with exhaust braking. Generally, an exhaust valve is opened during the first half of a compression stroke of a piston for exhaust gas recirculation. Opening of the exhaust valve during this time permits higher pressure exhaust gas from the exhaust manifold to recirculate back into the cylinder. The recirculated exhaust gas increases the total mass in the cylinder at the time of a subsequent braking exhaust valve event, thereby increasing the braking effect realized by the braking exhaust valve event.
Recently, varying the overlap between the time an exhaust valve is opened for exhaust gas recirculation and the time an intake valve is opened for intake has been recognized. Varying the overlap significantly reduces emissions of NOx (oxides of nitrogen). A system that varies the opening times of intake and exhaust valves is disclosed in U.S. Provisional Appln. No. 60/360,005, filed Feb. 28, 2002, the disclosure of which is incorporated herein by reference in its entirety.
U.S. Provisional Appln. No. 60/360,005 discloses a lash system for varying the amount of lash between the actuation piston and an exhaust valve to be opened by the piston, and independently controlling the exhaust valve opening and closing using levels of pressure and temperature in the exhaust manifold and engine cylinders, etc. Also disclosed is injection rate shaping.
There are many prior art systems that perform both exhaust gas recirculation and engine exhaust braking in a single system using multiple valve actuation, such as, e.g., U.S. Pat. No. 6,170,474 (Isreal), U.S. Pat. No. 6,082,328 (Meistrick et al.), U.S. Pat. No. 6,012,424 (Meistrick), U.S. Pat. No. 5,809,964 (Meistrick et al.), U.S. Pat. No. 5,787,859 (Meistrick et al.). However, there remains a need for, among other things, a method and system that performs both exhaust gas recirculation and engine exhaust braking using single valve actuation.
In preferred embodiments, a method and system is provided that performs both exhaust gas recirculation and engine exhaust braking using single valve actuation.
A first aspect of the preferred embodiments is generally applicable to a method of providing engine exhaust braking and exhaust gas recirculation for an engine having an exhaust manifold and a plurality of exhaust valves per cylinder during a four stroke engine cycle. The method comprises the step of (a) carrying out an exhaust braking event, comprising the steps of (a1) actuating a single exhaust valve beginning during a second half of a compression stroke continuing during a first half of an expansion stroke, and (a2) closing the exhaust valve beginning during a second half of an exhaust stroke, and the step of (b) carrying out an exhaust gas recirculation event, comprising the steps of (b1) reactuating the exhaust valve beginning during a first half of an intake stroke, and (b2) closing the exhaust valve beginning during a second half of the intake stroke, wherein the exhaust valve does not completely close during the exhaust braking event and the exhaust gas recirculation event.
A second aspect of the preferred embodiments is generally directed to a system for accomplishing engine exhaust braking and exhaust gas recirculation for an engine having an exhaust manifold and a plurality of exhaust valves per cylinder during a four stroke engine cycle. The system includes means for providing a valve train motion, and an actuation device operable to provide valve actuation of a single exhaust valve for an exhaust braking event and an exhaust gas recirculation event, wherein the exhaust valve is not completely closed during the exhaust braking event and the exhaust gas recirculation event. The exhaust braking event includes actuating a single exhaust valve beginning during a second half of a compression stroke and a first half of an expansion stroke, and closing the exhaust valve beginning during a second half of an exhaust stroke. The exhaust gas recirculation event includes reactuating the exhaust valve beginning during a first half of an intake stroke, and closing the exhaust valve beginning during a second half of the intake stroke.
The above and other features and advantages will be further understood from the following description of the preferred embodiment thereof, taken in conjunction with the accompanying drawings.